On Respiration 203 



of the blood. Nor indeed is the more frequent 

 respiration in violent exercise for the purpose of 

 cooling the blood which the motion heats. For in 

 violent exertions, be they so momentary that the 

 blood is not much warmed, there is certainly need of 

 more intense respiration than in the greatest state of 

 heat, and in fevers, that is when the blood boils more 

 and is as it were on fire ; so that respiration will be 

 seen to serve not so much for cooling as for motion 

 itself, as will be shown afterwards. 



But the prevalent opinion is that respiration is 

 necessary to life in order that the blood may be able 

 to pass through the lungs from the right ventricle of 

 the heart into the left. For the foetus in the uterus, 

 whose blood does not pass through the lungs but 

 through special ducts, does not need to breathe at all. 

 And this they say is the reason why there is not the 

 same necessity for breathing in the uterus as after 

 birth. 



But there is no reason why we should say that 

 Nature has constructed the lungs with so much skill 

 and labour only that the blood may pass through 

 them after birth, since it might pass by a shorter and 

 much less obstructed road through the same channels 

 it follows in the unborn foetus. Nay, it is the case 

 that the blood can pass through the lungs apart from 

 their motion. For if blood or any other liquid is 

 injected by means of a syringe into the pulmonary 

 artery of a dead animal it will pass readily enough 

 into the left ventricle of the heart. And indeed any 

 one can feel for himself that although respiration be 

 temporarily suspended, yet the pulse of the arteries in 

 the wrist is strong enough. But this would not be 

 the case if the blood were not passing at the moment 

 through the lungs to the left ventricle of the heart. 



